Antigen-specific T lymphocytes activation occurs through the clonally distributed T cell receptor (TCR), which is involved in thymocyte selection and peripheral T cell effector responses. How TCR-dependent lineage commitment from a common precursor occurs for the two major subsets of T cells, the CD4+ and CD8+ cells, remains unknown. Furthermore, the quantitative and qualitative relationships between receptor occupancy and signaling for differentiation are also poorly understood. This project uses cellular, biochemical, and molecular approaches to study differentiation of thymocytes and activation of T cells upon ligand engagement. Our previous studies suggested that thymocyte development could involve lineage choice that is independent of the MHC class involved in T cell recognition, followed by complete maturation (positive selection) of only those cells whose remaining expression of CD4 or CD8 matched the MHC class-specificity of the T cell receptor. Studies of additional gene targeted and transgenic mice support this conclusion, yet show that MHC recognition per se is critical to this step in development. These experiments also indicate that the transitional phenotype cells whose presence suggests lineage commitment are not the products of negative selection. These cellular studies will be critical for future molecular studies aimed at the development of a genetic understanding of the basis for T cell differentiation. We previously reported that some TCR ligands evoke only a subset of T cell effector responses, and that such variant ligands can selectively antagonize cytokine production induced by known receptor agonists. We have now identified additional peptide-MHC class II complexes with such partial agonist/partial antagonist properties. Preliminary studies on the biochemical events that accompany TCR recognition of such variant ligands suggest that they induce the generation of only a subset of the second messengers typically produced upon agonist recognition. These findings, in concert with our work on the molecular organization of T cell signal transduction complexes (Z01 AI 00349-11 LI), will help explain how protein-protein interactions result in antigen receptor- dependent second messenger generation, and how specific intracellular signals contribute to downstream gene activation events in T cells.